Combined frequency modulation radio transmitter and receiver



w. w. VOGEL 2,4085% SMITTER AND RECEIVER Oct. 8, 1946.

COMBINED FREQUENCY MQDULATION RADIO TRAN Filed June 21, 1945 5Sheets-Sheet l 5 in L m. @3322. 21022355; 55. a Q m 3923 INVENTOR.WlLLlAM W. VOGEL- ATTORNEY Oct. 8, 1946.

w. w. VOGEL COMBINED FREQUENCY MODULATION RADIO TRANSMITTER AND RECEIVERFiled June 21, 1943 5 Sheets-Sheet 2 FIG. 2

HIGH PASS NOISE AMPLIFIER 3O 8 REC AUDIO AMPLIFIER MUTING OSCILLATfiRECTIFIER 53 MICROPHONE 35 5 I2 I 36 l38al24 DISCRIMINATOR 27 INVENTOR.WILLIAM W. VOGEL ATTORN EY Oct. 8, 1946.

w. w. VOGEL 2,408,826

COMBINED FREQUENCY MODULATION RADIO TRANSMITTER AND RECEIVER Filed June21, 1945 s Sheets-Shet 5 F? no 20 saw 136 as TO MUTING' OSOILLAIQRLIIIIITER 26 AND RECT 33.

TO AFC 21 3mg;- MODULATOR l3 FIG. 5 RELATIVE DISORIHIINIKTOR OUTPUTVOLTAGE FIG. 4 I mmvrox i WILLIAM W. VOGEL BY I 2 3 W RELATIVE F-MVOLTAGE INPUT m NIOROVOLTS AT ANTENNA GROUND CIR. I9

Patented Oct. 8, 1946 CONHHNED FREQUENCY MODULATION RADIO TRANSMITTERAND RECEIVER William W. Vogel, Chicago, 111., assignor to GalvinManufacturing Corporation, Chicago, 111., a corporation of IllinoisApplication June 21, 1943, Serial No. 491,595

21 Claims. '1

The present invention relates to improvements in radio communicationapparatus and more particularly to improvements in combination radiotransmitter and receiver systems of the character use in police andmilitary communication work, for example.

Complete self -contained combination transmitter and receiver units,both of the portable and fixed position types, are now extensively usedin many forms of radio communication work and are especially useful intwo-way police and military communication work. Such units must ofnecessity be rugged, light in weight, and easily manufactured inproduction quantities at low cost. A unit of this type must also becapable of being easily and rapidly conditioned to operate either as atransmitter or as a, receiver, and should be easily tunable to transmitor receive at any desired carrier frequency within an allotted frequencyband. Other requirements of such a unit are that the receiving channelof theunit remain quiet during periods when the unit is conditioned forsignal reception but a selected signal is not being received, that theunit have sufficient signal radiating power and sufficient sensitivityof reception 'to permit high quality two-way communication to be heldover substantial distances, that the unit be capable of radiating asignal at a selected carrier frequency with a minimunl of drift in thecarrier frequency, that the receiving channel be automatically blockedagainst signal detection therein during signal transmission, and thatthe receiving channel of the unit be capable of maintaining itssensitivity in receiving a signal carrier having a drifting carrierfrequency.

In general, it is an object of the present invention to provide animproved combination radio transmitting and receiving system which meetsall of the requirements outlined above in a highly satisfactory manner.

it is another object of the invention to provide system of'the'frequency modulated type which meets all of the requirementsoutlined above.

it is a further object of the invention to provide a combinedtransmitting and receiving system which is provided with a receivingchannel of the superheterodyne type, and in which porti ans of both thetransmission and receiving channels ar used both during signaltransmission and signal reception, thereby to minimize the number ofcomponent parts of the system without sacriflcing desirable operatingfeatures.

' According to still another object of the invention, an improved methodis provided for producing a signal modulated carrier, for radiating thesignal at a particular carrier frequency, and for minimizingfluctuations in the carrier frequency.

In the illustrated embodiment of the invention, the transmission channelis provided with a mixer stage which follows th master carrier producingoscillator and utilizes a crystal controlled oscillator having aresonant frequency equaling the center intermediate frequency of thefirst intermediate frequency section of the receiving channel, in orderto permit signal transmission and reception at the same carrierfrequency without altering the tuning of any of the tunable stages ofthe system. More specifically, the master os-' cillator of thetransmission channel is coupled to the first mixer stage of thereceiving channel to operate as a, local oscillator during signalreception. and a mixer stage is provided in the transmission channel foreffecting radiation of a signal modulation carrier produced in thetransmission channel at the same carrier frequency as that which thereceiving channel is conditioned to receive.

According to another object of the invention, energy derived from thecrystal controlledbscillater of the transmission channel is utilized tocontrol the master oscillator so that variations in the center frequencyof the voltage developed at the output side of the master oscillator areminimized during signal transmission.

According to a still further object of the invention, energy derivedfrom the crystal oscillator of the transmission channel is also utilizedto block the receiving channel against signal detection therein when thesystem is conditioned for signal transmission.

In accordance with a further and more specific object of the invention,muting facilities are'provided for automatically blocking the audiosection of the receiving channel to prevent noise voltages which appearin this channel from being reproduced when the system is conditioned forsignal reception but a selected signal is not being received, and energyderived from the transmission channel is utilized automatically torender the muting facilities inactive to block the audio section of thereceiving channel when the system is conditioned for signaltransmission.

In accordance with still another object of the invention, a couplingpath is provided between the crystal controlled oscillator of thetransmission channel and the first mixer stage of the receiving channelto inject a carrier voltage into the receiving channel during signaltransmission for the stated purposes of blocking the receiving channelagainst signal detection during signal transmission, stabilizing thecarrier output frequency of the master oscillator during signaltransmission, and rendering the muting facilities inactive during signaltransmission.

It is another object of the invention to provide an improved radioreceiving system of the double superheterodyne type which is providedwith an oscillator of the crystal controlled type at the second mixerstage thereof.

According to a further object of the invention, the crystal controlledoscillator provided at the second mixer stage of the receiving channelis arranged to coact with the crystal controlled oscillator of thetransmission channel to minimize fluctuations in the carrier outputfrequency of the master oscillator during signal transmission.

It is a still further object of the invention to provide an improvedarrangement for electively rendering the transmission and receivingchannels active and inactive in a manner such that all of the facilitiesmentioned above are selectively, automatically and appropriatelyrendered active and inactive as the two channels are selectivelyconditioned for signal transmission and signal reception.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawings, in which:

Figs. 1 and 2, when laid end to end in the order named, illustrate acombined frequency modulated radio transmitting and receiving systemcharacterized by the features of the invention briefly referred toabove;

Fig. 3 diagrammatically illustrates the circuit arrangement of thecathode heaters of the electron discharge tubes included in the systemshown in Figs. 1 and 2;

Fig. 4 is a graph illustrating the noise and signal responsecharacteristics of the receiver; and

Fig. 5 is a circuit diagram illustrating a modification of the receivingequipment forming a part of the system shown in Figs. 1 and 2.

Referring now more particularly to Figs. 1 and 2 of the drawings, thereis illustrated, par-- tially in schematic form, a combination frequencymodulated radio transmission and receiving system which is well adaptedfor use as a complete portable unit and includes a transmitting sectionl8 and a receiving section H commonly coupled to an antenna groundcircuit l9 through a tunable antenna circuit l3. Briefly considered, thetransmitting section I 0 comprises a combination automatic frequencycontrol and reactance modulator stage l3, a tunable master oscillator H,a tunable frequency doubler network IS, a tunable transmitter mixer It,a power amplifier l1, and the tunable antenna circuit l8, connected intandem in the order named. The receiving section I of the systemcomprises the tunable antenna circuit l8, a tunable radio frequencyamplifier 28, a first mixer or converter stage 2|, a first intermediatefrequency amplifier 22, a second mixer or converter stage 23, a secondintermediate frequency amplifier 24, a first limiter 25, a secondlimiter 26, a frequency discriminator 21, an audio frequency amplifier28,

.and a loud speaker 29, all connected in cascade in the order named. Aspointed out below the system may be selectively controlled to operateeither as a transmitter or a receiver and, when conditioned foroperation, is set to operate as a receiver. In order to render the audiosection of the receiving channel inoperative to pass noise signalsappearing in this channel during intervals when a desired signal is notbeing received, muting or squelch apparatus is provided which comprisesa high pass filter network at coupled to the output side of thefrequency discriminator 21, a noise amplifier and rectifier 3|, a directcurrent amplifier 32 and a muting oscillator and rectifier section 33.These stages are connected in tandem in the order named, and respond tonoise voltages appearing in the receiver channel to impress a blockingbias voltage upon the audio amplifier 28 in the manner explained below.

More specifically considered, the transmitting section of the systemcomprises a microphone I! which is arranged to impress audio frequencyvoltages developed during operation thereof between the input electrodesof the combination frequency control and modulator tube 38 through acoupling network which comprises a microphone transformer 35, acondenser 36, and a resistor 31.

The space current path through the tube 38 is connected in shunt withthe space current path through the tube 40 of the master oscillator l4,and also shunts the tunable frequency determining circuit 4| of themaster oscillator I4. This oscillator is of the conventional tuned platecircuit type, the fresuency determining circuit 4| thereof comprising afixed inductance element 4|a whichis tuned to the desired resonantfrequency by means of the shunt connected fixed condenser 4|b and anadjustable tuning condenser 4|c. Operating potentials are supplied tothe anodes of the tubes 40 and 38 through a resistor 46 and theinductance element 4|a, a low impedance direct current blockingcondenser 4 Id being provided in the tunable circuit 4| in order toisolate this direct current path from ground. The tunable frequencydetermining circuit 4| of the oscillator I4 is regeneratively coupled tothe input electrodes of the tube 40 by means of an inductance element 42which is inductively coupled to the inductance element 4|. It is alsocoupled to the input electrodes of the frequency control and modulatortube 38 by means of the inductance element 42 and a. phase shiftingnetwork which includes the condenser 31a and the resistor 31b. Asuitable grid condenser 44 shunted by a grid leak resistor 43 isserially included in the input circuit of the oscillator tube I4 for thepurpose of maintaining the control grid of this tube at the properoperating potential with respect to the cathode of the tube.

The signal modulated carrier voltage developed across the tunablefrequency determining circuit 4|, is impressed between the inputelectrodes of a tube 41 included in the frequency doubler 5 through anetwork which comprises the coupling condenser 49 and a resistor 48.This tube is provided at its output side with a tunable frequencyselective circuit 52, which includes a fixed inductance element 52ashunted by a fixed tuning condenser 52b and an adjustable tuningcondenser 52c, and is tuned to a center frequency substantially twicethe center resonant frequency of the frequency determining network 4|forming a part of the master oscillator stage I4. Anode current issupplied to the tube 41 through a filter resistor 54 and the inductanceelement 52a, and the usual direct current isolating condenser 52d isprovided in the circuit 52 to isolate the anode current path fromground.

In accordance with the present invention, the

carrier voltage developed through operation of the tunable masteroscillator l4 and the tunable frequency doubler I5 is utilized as aheterodyning frequency source for converting a received. frequencymodulated radio frequency carrier into a correspondingly modulatedintermediate frequency carrier in the first mixer stage 2! of thereceiver channel ll. When, therefore, it is desired to utilize thesystem to transmit and receive signals at a fixed and preestablishedcarrier frequency without altering the tuning of the tunable circuitsinthe system incident to a change from transmission to reception, orvice versa, it is necessary to increase or decrease the output frequencyof the frequency doubler i5 by an amount equal to the value of theintermediate frequency utilized in the first intermediate frequencyamplifier section 22 of the receiver channel I I. Whether or not theoutput frequency of the doubler I5 is raised or lowered to provide thedesired frequency of carrier transmission, will, of course depend uponwhether the doubler output frequency is above'or below the particularcarrier frequency at which transmission is to be effected. In theparticular arrangement illustrated, a piezoelectric crystal 58 having aresonant frequency equal to the intermediate frequency utilized in thefirst intermediate amplifier 22 of the receiver is utilized to increase.the frequency of a transmited signal carrier above the carrierfrequency appearing across the frequency selective circuit 52 by anamount equal to the intermediate frequency utilized in the firstintermediate frequency amplifier 22. More specifically considered, thetunable circuit 52 is coupled to the input electrodes of the transmittermixer tube 63 through the shunt connected crystal 58 and condenser 59. Agrid leak and condenser network comprising the two resistors 55 and 56and a condenserfvl is provided for maintaining the proper bias potentialbetween the input electrodes of the mixer tube 63. For the purpose ofdriving the crystal 58 to maintain oscillation of the crystal at itsresonant frequency, a tuned circuit 69 is provided which is suitablydesigned to resonate at the same frequency as the crystal 58 andcomprises ,a fixed condenser 60a shunted by an adjustable inductanceelement 601). This network is included in the screen electrode circuitof the mixer tube 63 and also in the path comprising the'resistor 52over which the required operating potential is positively applied to thescreen electrode of the tube 63.

The mixer tube 53 is provided with a tunable frequency selective outputcircuit 64, whichcomprises a fixed inductance element E lashunted by thefixed condenser Mb and the adjustable tuning condenser this through thelow impedance direct current isolating condensers 64d and .Gde. Thiscircuit is normally maintainedtuned to a frequency which is equal totwice the output frequency of the oscillator it plus the resonant'frequency of the crystal 53, which latter frequency equals the firstintermediate frequency used in the receiver channel ll, The outputvoltage appearing across the circuit GA is impressed across the inputcircuit of the power amplifier I! through a coupling network whichincludes the condenser 65.

Referring now more in detail to the signal receiving channel H of thesystem, the first mixer stage 2! is illustrated as beingresistance-capacitance coupled to the output circuit of the tunableradio frequency amplifier 20 through a network which includes thecoupling condenser {wand tem is conditioned for reception,

a filter vresistor l3 whichiis quency utilized in the firstcondensersBI-b and 6 As indicated above, when the sys the tunable masteroscillator 14 and the tunable frequency doubler I5 are utilized as aheterodyning freresistor 69.

.quency source required to effect the desired carrier frequencyconversion in the first mixer stage 2 i. To this end, an inductanceelement 53 which is inductively coupled to the inductance element 52a ofthe frequency selecting circuit 52 is included'in the cathode-groundcircuit of the mixer tube Hi. The output electrodes of this tube arecoupled to a fixed tuned frequency selecting'circult .l'i whichcomprises a fixed condenser Hb shunted by anadjustable inductanceelement Ha and is tuned to the desired first intermediate frequency of4.3 megacycles, for example, Anode potential is supplied to the tube 10over a path which includes the inductance element lla and shunted by aby-pass condenser 12. The tuned output circuit H of the tube Hi iscoupled to the input electrodes of the first tube-in the intermediatefrequency amplifier 22 through a network which comprises the couplingcondenser'l land resistor 15.

The output side of the first intermediate frequency amplifier 22 iscoupled to the input electrodes of the mixer tube 16 provided at thesecond mixer or converter stage 23 in an obvious manner. This tube isprovided with output electrodes which are bridged by a frequencyselective circuit 8! tuned to the second intermediate frequency of 2.515megacycles, for example, and comprising a condenser 8H) shunted by anadjustable inductance element 81a. Anode potential is supplied to thetube 16 over a path which includes the inductance element Ma and afilter resistor 83 shunted by a by-pass condenser 82. The voltageappearing across the frequency selective circuit 8| is impressed acrossthe input side of the second intermediate frequency amplifier it througha network which comprises the coupling condenser'ad and a resistor 85.For the urpose of effecting the required carrier frequen'cyi onversionat the second mixer stage 23, the mixer tube section of the Pierce typewhich includes a piezoelectric crystal ll connected between the con- !6is provided with an oscillator trol and screen electrodes of the tube.This crystal has a resonant frequency of 6.815 megacycles which isgreater than the intermediate freintermediate frequencyamplifier 22 byan amount equal tothe intermediate frequency utilized in' the secondintermediate frequency am lifier 23. A suitable biasing networkcomprising the series connected resistors 18a and 18b shunted by thegrid condenser "i9 is provided between the input electrodes of the tube16 for maintaining the proper bias voltage between these electrodes.

Noise and signal voltages appearing at the output side of the secondlimiter 26 are introduced into the frequency discriminator 21. Brieflyconsidered, this discriminator comprises a tuned circuit 81, a pair ofdiode rectifier tubes 88 and '89, the space current paths of which arerespectively shunted by load resistors 90 and 9!, a radio frequencyby-pass condenser 93 having substantially negligible impedance tofrequencies of the order of the second intermediate frequency, and astabilizing condenser 92. More specifically, the resonant circuit '87serves totune the frequency discriminator network to .a center frequencyequal to the second intermediate 'frequency and comprises .a pair ofseries connected 810' which are shunted "byian 7 adjustable inductanceelement 81a. Preferably the last mentioned element is of the variablepermeability type being provided with an adjustable powdered ferrousmetal core, the position of which ma be changed to alter the inductanceof the element within the desired limits. The circuit constants of theresonant circuit 81 ar so chosen that the discriminator network isprovided with a band pass characteristic such that all' desired signalcomponents of a frequency modulated carrier appearing in the secondintermediate frequency channel 24, 25, 26 may be detected and impressedupon the input circuit of the audio amplifier 28. The voltage appearingacross the output side of the second limiter 26 is impressed upon thediscriminator network 21 through a coupling condenser 86 which isconnected at one side thereof to the junction point between the twocondensers 81b and 810. Audio frequency voltages detected throughoperation of the discriminator 21 appear across the condenser 93 and areimpressed upon the input side of the audio frequency amplifier 28through a coupling circuit which includes radio frequency decouplingresistor 94, an audio frequency filter comprising the resistor 35 andcondenser 96, an audio frequency coupling condenser 91, and a volumecontrol voltage dividing network comprising the two resistors 98 and Iand a direct current blocking condenser IOI. It will be understood inthis regard that the proportion of the availabl audio frequency voltageappearing across the series connected resistors 98 and I00 which isimpressed upon the input circuit of the audio frequency amplifier 28, isdetermined by the etting of the wiper 90 along the resistor 98.

As will be explained mor fully below, noise signals appearing in thesignal transmission channel of the receiver in the absence of a receivedsignal modulated carrier are passed through the discriminator 21 andappear as detected audio voltages across the condenser 93. Such detectedvoltage are impressed across the high pass filter network 30, and thosecomponents thereof having frequencies above the cutoff frequency of thefilter network are impressed between the input electrodes of the tub I06included in noise amplifier and rectifier 3I. More specificallyconsidered, the high pass filter 30 comprises a pair of seriescondensers I04 and I05 and a pair of shunt resistors I02 and I03, and isdesigned to pass those components of noise voltages which havefrequencies above the normal signal reproducing band of the receiver.The noise amplifier section of the tube I06 works into a noise rectifiercircuit which comprises the diode section of the tube and a loadresistor IIO. This rectifier circuit is coupled to the anode of the tubeI06 through a coupling condenser I08 which is of appropriate impedanceto pass any noise currents which may be transmitted through the highpass filter 30. Anode and screen potentials are supplied to the tube I06through the resistors I01 and I09, the second of which is by-passed toground through a condenser III.

Rectified noise voltages appearing across the load resistor IIO areutilized to control the bias between the input electrodes of the tubeII3 provided in the direct current amplifier 32. The initial orthreshold bias established between the electrodes of this tube isderived from a voltage dividing network, which comprises the seriesconnected resistors Ha, I IBD and H60 bridged across the availablesource of anode potential,

and is Provided with a .tap H1 adjustable along the resistor II6b toimpress a variable positive potential upon the control electrode of thetube II 3 through the filter resistor H5. The biasing circuits connectedbetween the input electrodes of the tube I I3 are by-passed for audiofrequency currents b means of a condenser II 4. Screen and anodepotentials are applied to the amplifier tube II3 through the resistor II8 and the resistors II 8 and H9, in series, respectively.

The direct current amplifier 32 as controlled by the variable biasvoltage derived from the load resistor H0, is utilized to control thestarting and stopping of the muting oscillator and rectifier 33. Thisstage of the muting or squelch apparatus comprises a dual purpose tube I20 having an oscillator section which includes a tuned frequencydetermining circuit I H connected between the output electrodes of thetube through a .by-pass condenser I22. The resonant circuit I2I is fixedtuned to a particular frequency of from 200 to 300 kilocycles andcomprises an inductance element I 2| a shunted by a tuning condenserI2Ib. It is regeneratively coupled to the input electrodes of the tube I20 by means of a feed back circuit which comprises an inductance elementI23 inductively coupled to the inductance element I2Ia and connected inseries with a parallel connected grid leak resistor I 24 and condenser I25 between the control grid and cathode of the tube I 20. Anodepotential is supplied to the tube I20 over a path which includes theinductance element I2I a and a resistor I26. The oscillator section ofthe tube I20 is coupled to the rectifyin circuit of the tube through acoupling condenser I29, and the indicated rectifying circuit seriallyincludes the diode rectifier section of the tube and the resistors I28,I32 and 18b. Any bias voltage appearing across the load resistors I28, I32 and 18b during operation of the oscillator and rectifier stage 33 isnegatively applied to the control grid of the first tube in the audiofrequency amplifier 28 over a path which comprises the resistor I21, theresistor I00 and the lower portion of the resistor 98.

In order to insure that the system will be speedily conditioned foroperation when cathode heating current is supplied to the cathode of thevarious tubes provided in the system, all of the tubes, with theexception of the discriminator diode 88, are of the filamentary cathodetype. The diode rectifier 88 must of necessity be of the indirectlyheated cathode type since the cathode thereof is, during operation ofthe discriminator 21, maintained at potentials substantially above thereference ground potential present upon the filamentary cathodes of theremaining tubes provided in the system. More specifically, the circuitarrangement of the cathodes provided in the various electron dischargetubes referred to above and also provided in the dia grammaticallyillustrated sections of the system, is shown in Fig. 3 of the drawings.In this circuit, reference characters corresponding to those used inFigs. 1 and 2, but having the differentiating subscripts a and b, areused to identify the relationship between the cathodes and therespective associated circuit sections, as shown in Figs. 1 and 2. Froma consideration of the circuit arrangement shown in Fig. 3, it will benoted that the various cathodes are effectively isolated at radio andaudio frequencies by means of the separating filter networks comprisingthe illustrated high impedance choke coils and the low impedance by-passcondensers. It

'9 will also be noted that current for energizing the various cathodesin the series-parallel circuit -is 'supplied'bya direct'current sourceI-SB-through the contacts of a manually operable on and ofi switch Thecathodeslfiaand-lia'of the electrondischargetubes respectivelyprovidedin the mixer IS and the power amplifier "i"? are arranged to beenergized in series with each other and with a suitable current limitingresistor it! through the contacts "of -a--manually operablepress-to-talk switch l 38. This switch is normally spring biased'to'itsopen circuit posi- 'tion and may be "utilized in the manner 'explainedbelow selectively toconditionthesystem for signal transmission or signalreception, as desired. It'is provided witha'pair of normally opencontacts 138a which are closed to connect the microphone 12 across theprimary winding of the transformer 35 only when the switch is operatedto condition the system for signal transmission. From'an inspection ofthecathode circuit arrangement, it 'will be apparentthatthis circuit hasbeen carefully arranged to utilize "the voltage drops across certain ofthe cathodes as bias voltages between certain of the other tubesprovided in the system. "For example, the voltage drop appearing acrossthe cathode "25a of the tube-provided in the firstlimiter stage 251simpressed :between the filamentary cathode 25a o'fthe tube ll in theproper direction to bias this cathode positively with respect to thecontrol grid of the tube. These bias voltages, asderived from thecircuit network shown in Fig.3, are appropriately indicated in Figs. theillustrated battery symbols, and the relationship between the respectivebattery symbols and the voltage drops across certain of the cathodesshown in Fig. 3 will'be readily apparent from a careful comparison ofthe circuit shown in Fig. 3 with that shown inFigs. 1 and2.

Preferably, the transmitter mixer tube263 is a pentode o'fthe well'knowncommercial 3A4 type, the frequency doubler tube 41 and the masterscillator tube pcnto'des, the automatic frequency control and reactancemodulator tube "idandthefirst mixer tube Ti] are commercial 'type 1L4pentodes, the

second mixer tube 16 is a commercial type 1R5,

pentagrid converter, the two diodes '88 and "89 are or" thecommercial'typelAB and'lSS espectively, thenoise amplifier and rectifiertube Hi5 andthe muting oscillator and rectifiertube F29 are commercialtype 185 pentodes, 'and'the direct current "amplifier tube i ii! is acommercial "type 1L4 pentode. Suitable screenjp'otentials are applied tothe tubes 63, 11, "til, '38 and T6 over direct currentpaths -whichrespectively "include thG'filtGfIfiSlStOlS 6'2, '56, 4ii,'-39and"8fi,-respectively. The potential applied" to the screen elecrode ofthe muting oscillator and 'rectifiertube 129 is controlled in -the'mannenmore fully explainedbelow to efiect the d'esired'staiting andstopping of the oscillator section of this tube. It will be understoodthat "the tuning elements of the various tunable circuits 'provided inthe system are gang controlled to be operated 'in unison, so thatfrequency alignmentbetween'the various resonant frequencies thereof 'ismaintained during each tuning operation. More specifically, the tuningelement s of the 'antenna circuit -i8, the tuning element of the-radiofrequency amplifier 20,-andthe adjustable condensers-64c, 52c and ile,respectively provided at the-tunablestages +6, 1 5-and l Maremechanically the input electrodes of l and 2 of the drawings by '66 arecommercial type 1T4 1 hey transmission at connected-in the mannerindicated by the dash line U,sothat allof the enumerated tuning elementsmay be'operated in'unison.

Briefly toconsider-the operation'of the system, it -will be understoodthat when the switch I36 is-operated'to its closed circuit position, thecathcdes of all tubes provided in the system, with the exception ofthe-cathodes tea and lla'of-the 'tubes-providedin the mixer it and thepower amplifier 41, are energized from the current source 135. If nowthe push-to-talk-swi-tch I38 =-is operated to itsclosedcircuit-position,the oathodes l dd and Haare alsoenergized. Due to the filamentarycharacter of the energized cathodes,

are rapidly heated to-electron emitting temperatures following-the-energization thereof.

When the two switches 136 andl'38 are thus operated, the system isconditioned for signa1 the particular carrier frequency established bythe tuning of the five tunable stages H5, l5, 1%, I'Land 48 of thetransmission channel. In thisregard it will be understood thatwhen-space current ilow through the tube 40 is initiated, the masteroscillator I4 starts to oscillate at a-carrier frequency which isprimarily determined by the setting of the tuning condenser Me and issecondarily determined by the mag-nitudecf the bias voltage between thecontrol-gridand-cathode of-the tube38. More generally considered,if-the'receiving channel H of the system is designed to operate with afirst intermediate frequency-M433 megacycles and signals are tobetransmitted and received-at a carrier frequency of 44.3 megacycles,thecondenser 'Mc is=soadjusted that with zero (bias upon the controlgrid of the-tube'3 8, the master oscillator l liwvill produce a carriervoltage having'a frequenc of -megacycles and-the other tunable circuitsof the transmission channel Ill are adjustedaccordingly. With thefrequency of the signal-carrier thus determined, an audio frequencyvoltage 7 developed through operation of the microphone i2 is impressedthrough the microphone-transformer 3'5 and the coupling condenser 36between the control grid and cathode of the modulator tube 38. The"resulting audio frequency variation of the voltage between thecontrol-grid and cathode of the tube '38 effectivelych-anges thereactance of the tunable frequency deter-mining circuit-'4 lof themaster oscillator l4 ata corresponding rate. In other words, varying thevoltage applied between the input electrodes of the tube -38 effectivelyserves to vary the tuning of the network in like manner, whereby thecarrier outputof the oscillator 14 is reactance modulated in accordancewith-the audio signal voltage impressed between the input electrodes "ofthe tube 38. This modulated carrier voltage isimpressed betweentheinput'electrodes of the tube' l'l of the frequency doubler i5 throughthe coupling condenser'49. Due to the action of the tube 41 indistorting the signal modulated carrier voltage and the action of thetunablefrequency selecting circuit-52 in selecting only'signal'modulatedcarrier components having twice thefrequen'cy-of the carriervoltagedeveloped at the output side of the oscillator '14, the modulatedcarrier voltage appearing across the output circuit of the doubler-llihas acarrier frequency'which is twice that of -the'oscillator-carrieroutput frequency, -i.- e. megacycles in the caseassumed'above. Thesignalmodulated carrier voltage appearing across the 'frequencyse-'lecting-circuit '52 is impressed between the input electrodes of thetransmitter mixer tube '63 over 1 l a path which includes the couplingcondenser 59 and the heterodyning piezoelectric crystal 58. Aspreviously explained, this crystal has a resonant frequency which isequal to the first intermediate frequency used in the receiving channelII of the system. Accordingly, this crystal, act- Dig in conjunctionwith the tuned circuit 60, functions to produce a carrier voltage whichis electronically mixed in the tube 63 with the carrier frequency outputacross the tuned circuit 52, so that a carrier is produced at the outputside of the mixer tube .63 having a frequency equal.

to twice the output frequency of the oscillator l4 plus the firstintermediate frequency. This carrier voltage is frequency modulated inaccordance with the audio frequency voltage applied to the inputelectrodes of the modulator tube 38. At the output side of the tube 63,this particular signal modulated carrier voltage is selected through theaction of the tuned frequency selecting circuit 64 and is impressedacross the input circuit of the power amplifier I 'I through thecoupling condenser 65. After being amplified by the amplifier II, thevoltage is transmitted through the tunable antenna circuit I8 andimpressed across the antenna ground circuit I9 for radiation.

Referring now more specifically to the function performed by the tunabletransmitter mixer I6, it is pointed out above that-the tunable frequencyselecting circuit 64 is tuned to respond only to a signal modulatedcarrier having a carrier frequency which is greater than twice theoutput carrier frequency of the oscillator I4 by an amount equal to theintermediate frequency utilized in the first intermediate frequencychannel 22 of the receiver. Since the carrier voltage appearing acrossthe tuned output circuit 52 of the frequency doubler I is used as aheterodyning frequency source at the first mixer stage 2| duringreception and this frequency is mixed with the frequency produced by thecrystal 58 to produce a frequency of carrier radiation which is equal tothe sum of the two frequencies, signal reception and transmission may beheld at the same carrier frequency. Thus if the crystal 58 has aresonant frequency of 4.3 megacycles, equaling the center intermediatefrequency used in the first intermediate frequency section of thereceiver chamiel II, and the tuning elements of the tunable stages I4,I5, IS, IT, I8 and 20 are adjusted by means of the adjusting element Uto a setting wherein the carrier output frequency of the oscillator I4is 20 megacycles and the carrier output frequency of the doubler I5 is40 megacycles, then the tunable stages I I, I8 and 20 are tuned to acarrier frequency of 44.3 megacycles. This of course means that if thetunable stages of two remotely located sets of the character illustratedare tuned for transmission and reception at the same carrier frequency,it is unnecessary to alter the settings of the tuning elements of eitherset when the direction of transmission between the two systems ischanged. Thus, the systems of the two sets may rapidly be altered fortransmission in either direction with a minimum number of manualoperations on the part of the persons using the respective sets fortwo-way communication.

As indicated above, the desired increase in the frequency of theradiated carrier over the carrier frequency appearing at the output sideof the frequency doubler I5 is provided through the action ofthepiezoelectric crystal 58. In considering the manner in which thiscrystal is driven at its resonant frequency, it is pointed out that atthis resonant frequency, the upper terminal of the tuned circuit 52 iseffectively at ground potential due to the low impedance of this 5circuit at the particular frequency in question. The resonant circuit 60which is coupled between the cathode and screen electrode of the tube 63is precisely tuned to the resonant frequency of the crystal 58. Due tothe electronic and capacitance coupling between the upper terminal ofthe tuned circuit 60 and the lower terminal of the crystal 58, asufficient driving voltage is applied across the crystal 58 through thetuned circuit 52 to maintain the oscillation of the crystal. Thiscoupling also serves to maintain the tuned circuit, 50 oscillating atits resonant frequency.

In order to condition the system for signal reception after signaltransmission has been'ef fected in the manner explained above, thepushto-talk switch I38 is released. Incident to the restoration of thisspring biased switch to its normal position, the cathodes I So and Na ofthe tube 63 and the tube provided in the power amplifier I! aredeenergized in an obvious manner. Thus, the transmitter mixer stage I 6and the power amplifier stage II of the transmitter channel III arerendered inactive without in any way interrupting or otherwise affectingthe operation of the preceding stages I3, I4 and I5. In this regard itis pointed out that when space current flow through the mixer tube 63 isinterrupted, the operation of the oscillator section of this tube, i. e.that portion of the tube input circuit which comprises the intercoupledcrystal 58 and resonant circuit 60, is arrested. Thus, no carriervoltage is produced in the transmission channel Ill having a frequencyapproaching the intermediate frequency used at either the first orsecond intermediate frequency sections of the receiving channel II.Accordingly, the continued operation of the three stages I3, I4 and I 5of the transmission channel II can in no way interfere with thereception of a selected signal modulated carrier.

Assuming that the system is conditioned for signal reception in themanner explained above, and that the tunable stages of the system areappropriately tuned to the center frequency of a desired frequencymodulated signal carrier, the 50 signal carrier voltage appearing acrossthe antenna ground circuit I9 is transmitted through the tunable circuitI8 and the coupling condenser 61 to the input side of the tunable radiofrequency amplifier 20. This voltage, as amplified 55 by the amplifier20, is mixed with the carrier output voltage of theirequency doubler I5,which output voltage is impressed between the cathode and control gridof the tubeIIi over a coupling path including the inductance element 53.It is 60 thus converted into a signal modulated intermediate frequencycarrier which is amplified through the first intermediate amplifier 22and impressed between the input electrodes of the tube 16 provided inthe second mixer stage 23.

In accordance with the present invention, the intermediate frequencycarrier output from the amplifier 22 is, in the second mixer stage 23,mixed with the carrier frequency produced through operation of theoscillator section of the 70 tube 16 as controlled by the crystal 11, sothat a beat frequency carrier, modulated with the signal voltage and ofthe desired second interme diate frequency, appears across the tunedoutput circuit 8|. This modulated carrier, as se- 75 lected through theaction of the tuned circuit III,

13' is transmitted through the condenser 84 tothe second intermediatefrequency amplifier 24 where it is amplified and transmittedsuccessively through the limiter stages 25 and 26 to the input side ofthe discriminator 21. In this discriminator the modulation components ofthe second intermediate frequency carrier, as represented by deviationsin the carrier frequency from the established center frequency, aredetected in the manner pointed out below. The detected signal voltageappears across the condenser 93, which condenser is possessed ofexceedingly low impedance at the center carrier frequency andexceedingly high impedance at the audio frequencies. This voltage isimpressed across the voltage dividing network comprising the resistors98 and 100 through the carrier frequency decoupling resistor 94 and theaudio frequency coupling condenser 91. The portion of thisvoltage whichappears between the wiper 99 and ground is impressed across the inputcircuit of the audio frequency amplifier 28 in an obvious manner. Theaudio frequency signal voltage as impressed across the input side of theaudio frequency amplifier and transmitted production.

Referring now more particularly to the operation of the discriminator21, it will be noted that this circuit is essentially a circuit two armsof which respectively include the condensers 81b and 810 of equalcapacitances. A third arm of th'e'bridge comprises the capacitiveimpedance of the diode 8.8. The fourth arm of the bridge comprises thecombined capacitive impedance of the diode 89 and the condenser :2. Theinductance element 81a is bridged between two terminals of the bridgecircuit and the frequency modulated signal voltage is applied to thecircuit across the other two terminals thereof. Since the load resistors90 and El have impedances far in excess of the capacitive impedances oithe diode legs of the bridge circuit at the frequencies involved, theymay be neglected in analyzing the circuit. Again the capacitance of thecondenser 93 is so much greater than that of. either diode leg of thecircuit, that this condenser may also be'neglected in analyzing thecircuit, With this bridge circuit arrangement the voltage appearing atthe output side of the discriminator is the difference between theabsolute values of the voltages to'ground at the upper and lowerterminals of the inductance element Ea. From an examination of thebridge, it will be understood that if the capacitance of the condenserSlb equals that of the condenser 810, which it does, and the capacitanceof the two diode legs of the circuit are equal, such that the bridge isbalanced, the currents respectively traversing the condensers 81b and810 are equal so that equal voltage drops appear across thesecondensers. Accordingly, no difierence between the voltages to ground isdeveloped at the upper and lower terminals of the inductance element81a, regardless of the frequency of the exciting voltage applied to thecircuit. In the actual circuit, however, the capacitance of the legwhich includes the diode 89 is greater than the capacitance of the legincluding the diode 88 by an amount equal to the capacitance value ofthe condenser 92, such that the bridge is unbalanced. Accordingly duringexcitation of the circuit, the current traversing the condenser 81cexceeds the current traversing the condenser 81b sothat a to the loudspeaker 29 for re- 28 is amplified in this amplifier four terminalbridge current is caused to how through theinductance element 81a.

The magnitude of this current obviously depends upon the reactiveimpedance of the inductance element 81a at the particular frequency ofexcitation, and the direction of current-flow is such that the voltagedrop across the condenser il'ib is enhanced and that across thecondenser iilc is decreased. It willbe understood, therefore, that bysuitably proportioning the impedance of the inductance element 81arelative to the reactive impedances of the condensers 81b and Bic at aparticular center frequency, to establish a given relationship betweenthe currents traversing the circuit elements 81a, 81b and 81c, theabsolute voltages between the upper and lower terminals of theinductance element 81a and ground become equal, In their relationship toeach other, however, these voltages are out of phase so that adifference voltage actually exists between the upper and lower terminalsof the circuit 87. This difference voltage is, of course, equal to thevector sum of the absolute voltages from the upper and lower terminalsof th inductance element 87a to ground. The particular frequency atwhich these absolute voltages become equal to balance the bridgerepresents the center frequency at which the voltage appearing at theoutput side of the discriminator between the cathode of the diode 88 andground becomes zero. In this regard it is pointed out that when thebridge is balanced so that the voltages from the upper and lowerterminals of the inductance element 81a to ground are equal, equaldirect voltages are produced across'the' load resistors 90 and 9|. Thesevoltages are opposingly combined in a direct current path through theinductance element 81a so that when equal, no direct voltage appearsbetween the cathode of the diode 88 and ground.

As the exciting voltage for the resonant circuit 8? is increased abovethe center frequency, due to the signa1 modulation thereof at an audiorate,

4 the reactive impedance of the circuit constants change to alter therelative magnitudes of the currents traversing the circuit elements 81a,81b and 810, so that the voltage from the upper terminal of theinductance element 81a to ground exceeds that between the lower terminalof the inductance element 81a and ground. Accordinely, a voltage whichis positive with respect to ground is produced between the cathode ofthe diode 86 and ground. If, on the other hand, the exciting frequencyfor the circuit 81 is decreased below the center frequency, the reactiveimpedances of the circuit constants change to alter the relativemagnitudes of the currents traversing the circuit elements Bid, 81?) and810 so that the voltage between the lower terminal of the inductanceelement 81a and ground exceeds that between the upper terminal of theinductance element 811; and ground. As a result, an output voltage whichis negative with respect to ground is produced between the cathode ofthe diode 8B and ground. It has been found that the extent or magnitudeof the discriminator output voltage varies in accordance with thedeparture of the exciting frequency from the center intermediatefrequency to which the discriminator network 21 is center tuned. It willbe understood, therefore, that if the frequency of the carrier appearingat the output side of the limiter 26 is frequency modulated inaccordance with a given audio signal, a corresponding audiofrequencyvoltage is accurately reproduced across the condenser 93 at the outputside of the discriminator 21.

To consider somewhat more fully the action of the condenser 92 instabilizing the operation of the discriminator network 27, it may bepointed out that if the impedances of the four legs of the bridgecircuit are perfectly balanced, changes in the exciting frequency willnot produce the desired differences of potential between the upper andlower terminals of the inductance element 81 and ground. By providingthe condenser 92 connected in the manner illustrated, however, therebyto insure that the over-all capacitance between the lower terminal ofthe inductance element 81a and ground exceeds that between the upperterminal of this element and ground, the desired circulating currentwithin the resonant circuit 81 will always be produced to insurestability of circuit operation. In this regard it is pointed out thatthe unbalancing or stabilizing condenser 92 may be connected eitherbetween the lower terminal of the resonant circuit 81 and ground orbetween the upper terminal of this circuit and ground. In either case,the desired operation of the network is produced. It is noted, however,that when a condenser 92 of appropriate capacitance value is connectedbetween the upper terminal of the circuit 8'! and ground, the directionof circulating current flow within the circuit is reversed. Accordingly,the polarity of the output voltage produced across the condenser 93incident to a given departure of the exciting frequency from the centerintermediate frequency is the reverse of that which is obtained for thesame frequency departure when the condenser 92 is connected between thelower terminal of the resonant circuit and ground.

If desired, one rectifying section of the improved discriminator 21 maybe combined with the audio frequency amplifier 28 in the mannerillustrated in Fig. of the drawings, wherein reference characterscorresponding to those used in Fig. 2 identify the same circuitelements. From an examination of the Fig. 5 arrangement,

it will be seen that the diode section of the tube 89 is utilized as oneof the rectifying aths of the discriminator, and that the cathode, anodeand three grids of the tube are used to amplify the audio frequencyvoltage which is developed between the wiper 99 and ground duringreception 50 of a selected signal. This audio voltage is transmitted tothe loud speaker I29 for reproduction through a coupling transformerI39. The manner in which the audio section of the tube is blocked underthe control of the muting oscillator 33 and mode of operation of thediscriminator 2'! are exactly the same as explained herein withreference to the system shown in Figs. 1 and 2. In fact, the circuit ofFig. 5 may be directly substituted for the discriminator 2! and theaudio 60 frequency amplifier 28 in the system of Figs. 1 and 2 toperform in the same manner, when the indicated connections are madebetween this circuit and the limiter 26, the high pass filter 30, thetransmitter mixer I6, the muting oscillator and rectifier 33, and themodulator stage I3.

Automatic frequency control As previously indicated, provisionsincluding the ground, to be produced between stantially constant valuewhich substantially equals the center intermediate frequency to whichthe resonant circuit of the first intermediate frequency section of thereceiving channel I I are tuned. The purpose of this arrangement is tocorrect for any drift in the output frequency of the oscillator I 4 orin the center frequency of the received signal carrier. In this regard,it is noted that regardless of the settings of the tuning elementsprovided in the tunable stages I8 and 20 of the receiving channel II,these stages are broadly tuned to the center carrier frequency whichcorrespond to the settings of the tuning elements, so that irrespectiveof any drift in the center frequency of the received carrier allmodulation components of the received signal are passed through thesestages of the receiving channel. In a similar manner, the fixed tunedstages of the first and second intermediate frequency sections of thechannel II are somewhat broadly tuned in order to permit, within limits,deviations in the center carrier frequencies appearing therein withoutcutting off the modulation components of the frequency modulatedcarriers which are transmitted therethrough. It will be understood,therefore, that by providing the improved automatic frequency controlarrangement described below, any drift in the output frequency of theoscillator I4 or in the center frequency of a received signal carrier issubstantially corrected in so far as the intermediate frequency sectionsof the receiving channel and the discriminator 21 are concerned.

Briefly to consider the manner in which the output frequency of theoscillator I4 is automatically controlled, it may be assumed that thecenter frequency of the received signal carrier starts to drift to avalue higher than the center frequency to which the resonant circuits ofthe tunable stages I8 and 20 are tuned, or that whil this center carrierfrequency remains constant, the output frequency of the oscillator I4starts to drift from an established value to a lower value. As a resultof the frequency drift and regardless of where it originates, the centerfrequency of the carrier transmitted through the first intermediatefrequency amplifier 22 increases to decrease the center frequency of thecarrier transmitted through the second intermediate frequency stages 24,25 and 26. As will be apparent from the above explanation, thisdeparture in the exciting frequency of the tuned circuit 8! from thecenter frequency to which this circuit is tuned, causes a bias voltage,which is negative with respect to the cathode of the diode D8 andground. This bias voltage is negatively applied to the control grid ofthe modulator tube 38 over a path which includes the radio frequencydecoupling resistor 94, the audio frequency decoupling resistor 95 andthe filter resistors l3| and 31. At this point it is noted that theaudio frequency filter comprising the decoupling resistor 95 and theby-pass condenser 96 prevents the audio frequency components of thevoltage appearing at the output side of the discriminator 2! from beingimpressed between the input electrodes of the modulator tube 38. Thisfilter also prevent audio frequency voltages developed during signaltransmission by the microdiscriminator 21 and the modulator stage I3 of7 phone I2 from being impressed upon the input the transmission channelI, are made in the system for automatically adjusting the outputfrequency of the frequency doubler l5 so that the difference betweenthis frequency and the center frequency of a-selected carrier is held ata sub 28 through produces a corresponding. decrease in therate ofincrease of the negative bias voltage app-lied between? the control gridand cathode of the modulator tube 38. The bias applied to the tube 38continues to increase at a constantly decreasing rate until it isbalanced by the center frequencies of the signal carriers traversing thefirst and second intermediate frequency section of the receiving channelI Lat which point the center frequencies and the bias remain balancedagainst each other. If the circuit constants of th system areproperlychosen,'the:bias.voltage will in. each instance be stabilized at a,value such that the center frequencies of the signal carriers traversingthe first and second intermediate sections of the receiving channel IIwill be held at values which closely approximate the center frequenciesat which these sections of the receiving channel and thediscriminator'21 are designed to operate.

If the center frequency of a selected signal carrier drifts to a valuebelow the center frequency to which the stages I8 and 20 are tuned, orthe output frequency of the oscillator I-l drifts from its establishedvalue to a higher value, the center frequencies of the signal carrierstraversing the iirst and second intermediate frequency sections of thereceiving channel II are decreased and increased respectively. As aresult, a positive bias voltage appear-sat the output side of thediscriminator 27 which is applied to the tube 38 to produce a decreasein the output frequency of the oscillator'l'd. Thecenter frequencies ofthe signal carriers traversing the intermediate frequency sections ofthe receiving channel are increased and decreased accordingly. Thus, thefrequency correcting action proceeds in the exact manner explained aboveuntil a'point of stability is reached at which the center frequency ofthe signal carrier voltage impressed upon the discriminator network 21closely approximates the center frequenc to which the resonant circuit8'! is tuned.

Operation of the muting apparatus Referring now more particularly to themanner in which the audio section of the receiving channel I I is mutedor squelched during periods when the system is conditioned for operationbut is not being used either for signal transmission or reception, itmay be pointed out that at all times when the system is conditioned forreception but is not receiving a desired signal, noise signal voltagesappear in those stages of the receiver channel which precede thediscriminator 21. These Voltages are transmitted through theintermediate frequency and mixer stages Of'the channel I I and aredetected by the discriminator 21 to appear as audio frequency voltagesat'the output side .of the discriminator. They may be produced as aresult of thermal agitation within the tubes provided in the receivingchannel, shot effects,

extraneous noise voltages "appearing across the antenna-ground circuitI9, or'by physical shock decrease in the level of the noise developedacross 18 tolthe :circuit elementsprovided in the receiving channel.:Regardless of the origin thereof, howeverythe noise signals aremanifested as audio frequency voltages acrossthe output side of thediscriminator which, in the absence of the muting apparatus .iprovidedin the system, would be passed through the audio frequency amplifier 28.to the loud speaker 129 for reproduction,

More specifically considered, the noise response .of the receiver isgraphically illustrated in Fig. 4

noted that.when no signal carrier is being received, the noise voltageappearing at the output side of the discriminatorZl is high and that themagnitude ofthisvoltage is sharply reduced in response to theapplication of a selected signal carrier to the antenna-ground circuitI9. The voltage which accompanies the transmission of a selected signalthrough the receiving channel II, is largely effectedlin the amplitudelimiters 25 and 26.

To consider the action of the muting apparatus, it is pointed out thatthe noise voltage appearing between the cathode of the diode 88andground at the output side of the discriminator 2'! is impressed uponthe input sideof the highpassfilter '30. This filter acts to pass onlythose components of the noise voltage having frequencies above .the

normal signal reproducing band of the receiver. For example, this filtermay be designed to pass frequencies above 20 kilocycles. The noisevoltage appearing across the output side of the filter 30 is impressedbeteween the input electrodes of the noise amplifier and rectifier tubeI06 and appears in amplified'form across the coupling condenser I08 andthe diode section of the tube IE5 in series. diode section of the tubellldadirect voltage is Due to the rectifying action of the producedacross the load resistor I Ill which Varies .in magnitude in accordancewith the magnitude of the noise voltage impressed between the inputelectrodes of the tube I08, This direct'voltage,

i. esthatacross the resistor I I0, is negatively applied to the controlgrid of the direct voltage amplifier tube I 13 through the resistor I I2in op position to thefixed bias voltage normally positively applied tothe control grid of the tube II-3 through the resistor H5. The negativevoltage appearing across the resistor III) so greatly predominates overthat positively applied to the control grid of the tube H3 that thistube is biased beyond its space current cutoifpoint. Accordingly, thevoltage drops across th two resisters II8 and H9 are sharply decreasedto very low values, with the result that the full voltage 'of theavailable-source of anode current is positivelyapplied to the screenelectrode of the oscillator :and rectifier tube I20. Theapplication ofthis voltage to the screen electrode of the tube I26 initiates theoperation of the .oscillator section of .this tube, so that anoscillatory voltage is the series connected coupling condenser I29 andthe space current path between the diode electrodes of the tube. Due tothe action of the'diode section of the tube I20 in rectifying theoscillatory voltage, a direct bias voltage is produced across comprisingthe series resistors I28, I32 and 18b. This bias voltage :is negativelyapplied tothe controlgrid-of the first tube provided in the audiofrequency amplifier the diode load circuit 28, over a ipath whichincludes the resistors I21 and I and the encircuited portion of theresistor 98, The magnitude thereof is sufiicient to bias the first audiofrequency amplifier tube beyond cutoff, whereby the noise signals areprevented from being transmitted through the audio channel of thereceiver to the loud speaker 29 for reproduction.

As will be apparent from further consideration of the curve shown inFig. 4 of the drawings, when a selected signal carrier of substantialmagnitude appears across the antenna ground circuit I 9, the limitersand 26 function sharply todecrease the noise voltage developed at theoutput side of the discriminator 21. This produces a correspondingdecrease in the bias voltage developed across the load resistor IIU.When the negative bias applied to the control grid of the tube II 3 isthus reduced to a low value, the current flow through the resistors H8and I I9 and the space current path of the tube I I3 is sharplyincreased to produce a corresponding increase in the voltage dropsacross the two identified resistors. As a result, the voltage which ispositively applied to the screen electrode of the oscillator andrectifier tube I20 through the two resistors H8 and H9 is sharplydecreased to a value such that operation of the oscillator section ofthis tube cannot continue. When the production of an oscillatory voltageacross the space current path of the tube I20 is thus arrested, thenegative bias voltage across the. rectifier load circuit resistors I28,I32 and 18b is reduced to zero, permitting the normal negative biasvoltage as developed across the resistor 78b to be impressed upon thecontrol electrode of the first tube provided in the audio frequencyamplifier 28. When this amplifier tube isthus unblocked or biased to anormal value, the audio section of the receiving channel is renderedoperative to amplify the audio frequency components of the receivedsignal and to transmit the same to the loud speaker 29 for reproduction.

From the foregoing explanation it will be understood that normally, i.e. when the system is conditioned for signal reception, the noisesignals appearing in the receiving channel II are utilized to completelyblock the audio section of the receiving channel against thetransmission of noise signals to the loud speaker 29. Morespecifically,the component circuit elements. of the muting apparatus should be sochosen that in the absence of a desired signal, the negative biasvoltage developed at the upper terminal of the resistor I28 isapproximately 20 volts. To this end, from to volts must be positivelyapplied to the screen electrode of the oscillator and rectifier tube I20when a tube of the commercial 1S5 type is employed in the oscillator andrectifier stage 33. Further, the component circuit elements of themuting apparatus should be such that when a selected frequency modulatedcarrier is received having a magnitude exceeding a predetermined lowvalue, the voltage positively applied to the screen electrode of thetube I20 is dropped to approximately 20 volts such that operation of thoscillator section of the tube I20 is arrested. In the absence of anoscillatory voltage between the anode and cathode of this tube, the onlynegative bias voltage applied to the control grid of the first tube inthe audio frequency amplifier 28 is that developed across the grid leakresistor 18b, which voltag is of the orderof one volt. 4

When the apparatus is designed to have the characteristics justdescribed, the audio channel of the receiver will at all times remainblocked during periods when a selected signal is not being received andwill. be automatically unblocked when a selected signal is transmittedthrough the receiving channel of the system to the discriminator 2'! fordetection. In this regard it will be understood that since the high passfilter 30 will only pass frequencies outside of the normal signalreproducing frequency band of the receiving channel, the mutingapparatus is not responsive to the audio frequency components of areceived signal carrier and thus this apparatus i prevented fromblocking the audio section of the receiving channel against thetransmission of detected signal voltages to the loud speaker 29.

Blocking the receiving channel during transmission In considering themanner in which the receiving channel I I is blocked againstreproduction of the signal components of the modulated carrier radiatedduring operation of the transmission channel II, it is pointed out thatin accordance with the present invention, the equipment is deliberatelydesigned and is physically so arranged that a relatively large amount ofstray capacitance couplin exists between the circuit elements providedin the input and output circuits of the first mixer 2|, and theelectrodes of the crystal 58 and the circuit conductors connecting theseelectrodes with the input electrodes of the tube 63 and the terminals ofthe tuned circuit 52. More specifically, the electrodes of the crystal58, the elements of the tuned circuit 60 and the circuit elementsforming the input circuit for the mixer tube 70 are unshielded; and thecontrol grid of the tube III is spaced approximately one inch from thecircuit conductor which connects the control grid of the tube 63,

the lower electrode of the condenser 59 and the lower electrode of thecrystal 58. Energy derived from the oscillator section of the mixer tube63 is also delivered to the cathode-grid circuit of the mixer tube 10through the coupling between the two inductance elements 52a and 53. Theresulting coupling path is indicated by the broken line C which extendsbetween the two stages I6 and 2I.. .With this arrangement and during signal transmission, when the crystal 58 and the tuned circuit 60 areoscillating, a strong unmodulated carrier voltage appears at the outputside 'of the first mixer 2|, having a frequency equal to the centerfrequency to which the resonant circuits of the first intermediatefrequency section of the receiving channel are tuned. This strongcarrier voltage as transmitted through the first intermediate frequencyamplifier 22, th second mixer 23, the second intermediate frequencyamplifier 24, and the two limiter stages 25 and 26, to the input side ofthe discriminator 21, effectively blocks the enumerated stages of thereceiver against the transmission of the signal modulated carrier whichis impressed upon the input sid of the tunable radio amplifier 20through the condenser 61. More particularly, the blocking carriervoltage which appears across the output side of the first mixer 2I inthe receiver, as a result of the coupling path provided by thestraycapacitance coupling between the circuit elements of this mixer and thecircuit elements associated with the crystal 58,'exceeds by severaltimes the modulated signal carrier which appears at the input the mixer2!, those stages of canoe-see .21 side of the mixer 2| dueto thecouplinggbetween the transmitting and receiving channelsthroug thecondenser 67. :Since the carrier voltage as derived from the crystal 58so greatly predominates over that transmitted through th tunable radiofrequency amplifier 20 to the input side of the receiver which followthe mixer 2| are efiectively blocked against the transmission of thesignal modulated carrier to the discriminator. 27. Thus,.the.loudspeaker 29 is prevented'from reproducing the audio frequency voltagedeveloped through operation :of .the microphone 12 when the system isconditioned for transmission.

Stabilizing the carrier frequency daring transmission In accordance withanother feature of the present invention, the crystal 58 is alsoutilizedin conjunction with th fixed tuned stages of the receiver to set theradiated carriercenterfrequency so that this frequency cannot be changedby the discriminator 21 and is maintained-at desired value. Thus, due tothe capacitance coupling between the circuit elements of the first mixer'21 and the circuit .elements of the fixed tuned crystal 58 and'resonantcircuit til, a carrier havinga frequency exactly equaling the firstintermediate frequency is injected into thefirst intermediatefrequency'section of the receiving channel. This carrier is mixedWiththe frequency'produced by the oscillater section of the secondmixertube'lfi .to produce .a carrier in the second intermediate frequencysection of the receiving exactly equals the center frequency to whichthe resonant circuitL81 is tuned. When thiscarrier voltage is applied'tothe discriminator.2.l,.the bias voltage appearing at the output sideofthe discriminator between the lower terminal of the resistor 85 andground is reduced to anegligible or zero value. Moreover, since thecrystal 58 and the crystal ll whichcontrols the oscillator section ofthe second mixer tube I5 are invariably fixed to oscillate at setfrequencies, the negligible bias voltage appearing at th output side-fthe discriminator 21 cannot be changed or alteredin the slightest degreeduring transmission. Accordingly, the bias applied to the inputelectrodes of the modulator tube 38 through the resistors i3! and 31 isheld at a fixed negligible value during transmission with the resultthat the modulator oscillators l3, M are infiexibly set to produce asignal modulated carrier voltage having a'fixed center frequency. Thus,the crystal 58, acting in conjunction with the stages-HA2,23,24,2-5, 26andZ'l of the receiving channel ii, functions to stabilize the centerfrequency of the radiated signal carrier at the definite and fixed valuedesired. This center frequency value can, however, be altered byadjustment of the adjusting element U to alter the settings of thetuning elements provided in'the tunable stages [4, i5, 1-6, H and E8 ofthe transmission channel, but once th desired value is established it ismaintained by the crystal 58 in'the manner just explained.

Controlling the mating apparatus to unblock the audio frequency sectionof the receiving channel during signal transmission substantially theexact In accordance with another feature of the present invention, thestrong carrier injected-by the crystal 58 and its associatedcircuit-elements into the first intermediate frequency channel of thereceiving channel H through the channel which i --cap.acitance couplinglbetween 1the circuit :ele- .ments associated withitheicrystalzfl'andzth qcir- .cuit: elements :ofathe first :mixcr. 2 his also; utilized--l]0'lCOIltI'O1".-t'hB mutingapparatusesoxthat -the audio 5 .section-.23 :of :the'zrreceiving channel :11 ;is ;un- ::blocked cor :ren-deredvactive :during :signalxtrans- -mission. iMorezspecifically, this:carri'erEhas1the same reflect, in .so .faras :the :reduct-ion of74110158 voltages at the .outputrside of the discriminator 19 21 isconcerne1d,..as does the application ,ofisa strong signal of a' selected-center ,carrier Efrequencyaizo the 'antennargroundcircuit J9..,:Accordingly, andas willwbezapparentr by reconsider- ..ing the I curveshown ::in Eig. A 1 of the drawings,

when the. strong :carrier 'is. injected into rthefirst intermediatefrequency section ".of the ;,receiving channel, theincisevoltage:developed. betweenethe cathode of the. tube :88' and igroundizatthezoutput side of the'discriminator 21.drops'tovanegligible value. -.Asa result, the" oscillator :SBQtiOIlwOfithG tube I20 stops oscillating,for ireasonsrexplained above, and the negative blocking bias is removedfrom the control grid of the firsttube inrtheaudio frequency amplifier128. Thus, this ,amplifiergis rendered operative, and man: if desired; 1used to amplify 1 and transmit to ethe loud-speaker .129 forreproduction, any side tone voltage-suitably derived from the audiochannel 10f the "transmission channel H]. l

:From the foregoing explanation it will ibe apparent that an improvedarrangement, including the crystal controlled oscillator section :ofzthe mixer tube 63, the coupling epathbetweenithis oscillator sectionand .the first mixer istageizi and the crystal controlled oscillatonsection ofithe mixer tube 16, is provided. for minimizing .fiuctuationsin the output carrier. frequency. of .thesmaster oscillator l4 duringsignal transmission. These portions of .the .system are also utilized toperform other functions, such .that unnecessary .duplica- -tion .ofcircuit elements is avoided. :ThllS, the crystal controlled oscillatorzsectionaof ;the mixer .tube .63 and the coupling path'between thisoscillator' section .and the first mixer stage 2 l, func- -tion to block:the :receiving :channel againstisignal detection therein:duringl'signalitransmission. All

three of the identifiedzportions of the assist-em .coact to .controlthemutinglapparatusrso that-the audio section of the receiving channel :is,unbloclced during signal transmission. The -importanceof providing acrystalcontrolledpscillator section in :the second .mixenstageof: thereceivin channel will befully. apparent iniviewpf the;pre- "cedingiexplanation. :Thus, sby-.using.;;a:1fixed :output frequency oscillatorof 1this character sat .a point in the control channel connecting thecrystal oscillatorsection of the mixer [6 with the input electrodes ofthe modulator :tube 38, itubecomes impossible to introduce anydrift'intozthe center carrier frequency appearing at the input side ofthe discriminator-'21. This WOll'ldiIlOil-hfi the case if an oscillatorof the (tank circuit type were used at the second mixerstagmofthereceiving channel.

While one embodiment of the invention has been disclosed, it will beunderstood that-various modifications may be made therein, which arewithin the true spirit and scope of the invention.

I claim:

1. In a combined radio transmitting -and receiving system which includesmeans for selectively conditioning said system'for'si'gnaltransmissionor reception, a receiving channel, a-signal responsive device operativein response to the 7 transmission of areceivedsignal through saidoscillator for'controlling said master oscillator through a section ofsaid receiving channel to govern the output frequency of said masteroscillator and for controlling said muting means through the samesection of said receiving channel 'to render said muting meansinoperative to prevent noise or signal voltages from being transmittedto said signal responsive device.

2. In a combined radio transmitting and receiving system which includesmeans for selectively conditioning said system for signal transmissionor reception, a receiving channel, a signal responsive device operativein response to the transmission of a received signal through saidchannel, muting means responsive to noise voltages appearing in saidchannel in the absence of a received signal and having frequenciesdifferent from the frequencies of received signals transmitted I throughsaid receiving channel to prevent said signal responsive device fromresponding to the noise voltage, a transmission channel including amaster oscillator, and means associated with said transmission channelfor controlling the output frequency of said master oscillator and forrendering said muting means inoperative to prevent noise or signalvoltages from being transmitted to said signal responsive device.

3. In a radio receiving system which includes a receiving channelprovided with an audio frequency section, a signal responsive deviceoperative in response to a signal transmitted through said channel,means responsive to noise voltages appearing in said channel in theabsence of a received signal modulated carrier and having frequenciesdifferent from the frequencies of received signals transmitted throughsaid receiving channel for blocking theaudio section of said channel andresponsive to a received carrier for unblocking said audio section, andmeans for injecting a carrier voltage into said channel at a pointpreceding said audio section, thereby to block a portion of said channelpreceding said audio section and to control said last-named means sothat said audio section is unblocked.

4. In a radio receiving system which includes a receiving channel and asignal responsive device operative in response to a Signal transmittedthrough said channel, means responsive to noise voltages appearing insaid channel in the absence of a received signal and having frequenciesdifferent from the frequencies of received signals transmitted throughsaid receiving channel for rendering said signal responsive devicenonresponsive to the noise voltages, and means for blocking said channelagainst the transmission of a received signal to said signal responsivedevice and for concurrently rendering said lastnamed means ineil'ective.

5. In a combined radio transmitting and receiving system which includesmeans for selectively conditioning said systemfor signal transmission orreception, a receiving channel including an audio frequency section,meansassociated with said channel for blocking. said audio sectionagainst the transmission of noise voltages appearing in said channel inthe absence of a received signal, and means operative during signaltransmission for automatically controlling said last-named means to opensaid audio section and for concurrently blocking against signaltransmission a portion of said channel preceding said audio section.

6. Ina combined radio transmitting and receiving system which includesmeans for selectively conditioning said system for signal transmissioncept a receiving channel, a signal responsive device operative inresponse to a signal transmitted through said channel, means associatedwith said channel for rendering said signal responsive devicenonresponsive to noise voltages appearing in said channel in the absenceof a received signal, and means for automatically rendering saidlast-named means ineffective during signal transmission.

7. In a combined radio transmitting and receiving system which includesmeans for selectively conditioning the system for signal transmission orreception, a receiving channel including a mixer stage followed byintermediate and audio frequency sections and provided with at least onetunable frequency selective stage, a transmission channel provided witha plurality of tunable stages and including a tunable carrier producingoscillator which is coupled to said mixer stage to operate as a localoscillator during signal reception, means associated with one of saidchannels for adapting said system for signal transmission and receptionat the same carrier frequency without altering the setting of any of thetunable stages of either chamiel, and means controlled by saidlast-named means for normally blocking the audio section of saidreceiving channel and for opening said audio section during signaltransmission.

8. In a combined radio transmitting and re ceiving system which isadapted to be selectively conditioned for signal transmission or signalreception, transmission and receiving channels, a

plurality of coupling paths extending between said channels at pointsintermediate the ends thereof, means including one of said paths fortransferring energy from said transmission channel to said receivingchannel during signal reception, and means including another of saidpaths for transferring energy from said transmission channel to saidreceiving channel to block said receiving channel against signaldetection therein when said system is conditioned for signaltransmission.

9. In a combined radio transmitting and receiving system which isadapted to be selectively conditioned for signal transmission or signalreception, a receiving channel including a mixer stage followed by anintermediate frequency section, a transmission channel provided with atransmission mixer stage preceded by a carrier producing oscillatorwhich is coupled to aid first-named mixer stage to operate as a localoscillator during signal reception, a second oscillator included in saidtransmission mixer stage and tuned to resonate at a frequency equal tothe center frequency to which the intermediate frequency section of saidreceiving channel is tuned, a coupling path for injecting an oscillatoryvoltage derived from said second oscillator into the intermediatefrequency section of said receiving channel during signal transmission,thereby to block said receiving channel against signal detectiontherein, and means for prevent- 25- ing said voltage from'being injectedinto themtermediate. frequency section of, said receiving channel whensaid system is conditioned for sig nal reception.

10. In a combined radio transmittingand receiving system which isadapted to be selectively conditioned for signal transmission or'signalre ception, a receiving channel including a mixer Stage followed.by an intermediate frequency section, a transmission channel providedwith a transmission mixer stage preceded by a carrier producingoscillator whichis coupled to said first named mixer stage to operate asa local oscillator during signal reception, a second oscillator.included, in said transmission mixer stage andtuned to' resonate at afrequency equal to the center frequency to which the intermediatefrequency section of said receiving channel is tuned, and a couplingpath for injecting an oscillatory voltage derived from said secondoscillator into the intermediate frequency section of said receivingchannel during signal transmission, the energy transferred over saidcoupling path being sufficient to block said receiving channel againstsignal detection therein.

11. In a combined radio transmitting and receiving system which includesmeans for selectively conditioning the system for signal transmission orreception, a receiving channel including a mixer stage followed by anintermediate frequency section and provided with at least one tunablefrequency selective stage, a transmission channel provided with aplurality of tunable stages and including a tunable carrier producingoscillator which is coupled to said mixer stage to operate as a localoscillator during signal reception, means associated with one of saidchannels for adapting said system for signal transmission and receptionat the same carrier frequency without altering the tuning of any of thetunable stages of either channel, and means controlled by saidlast-named means through a portion of said receiving channel forpreventing variations in the carrier frequency of a transmitted carrier.

12. In a combined radio transmitting and receiving system which includesmeans for selectively conditioning the system for transmission orreception, a transmission channel including an oscillator and a mixerstage arranged in tandem in the order named, a crystal controlledoscillator included in said mixer stage to convert the output from saidfirst named oscillator into a carrier having a frequency difierent fromthe output frequency of said first-named oscillator, a receivingchannel, and means including a portion of said receiving channel andsaid crystal controlled oscillator for holding the frequency ofthevoltage appearing at the output side of said firstnamed oscillatorsubstantially constant.

13. In a radio transmitter, a transmission channel including amodulator, a carrier producing oscillator and a mixer stage intercoupledin tandem in the order named, a crystal controlled oscillator includedin said mixer stage to convert the modulated carrier output from saidfirst-named oscillator into a correspondingly modulated carrier having acarrier frequency different from the carrier frequency at the outputside of said first-named oscillator, and means including a controlchannel extending from said crystal controlled oscillator to saidmodulator and having a frequency discriminator therein for controllingsaid first-named oscillator so that the carrier frequency appearing atthe output side of saidfirst-named oscillator is held substantiallyconstant.

14-. In a radio transmitten, a transmission cluding said crystalcontrolledoscillator and a frequency discriminator controlled therebyfor controlling said first-named oscillator-to governthe outputfrequency of said first-named oscillator.

15. In a combined radio transmitting and receiving system which isadapted to be selectively conditioned for signal transmission or signalreception, a transmission channel including a carrier producingoscillator, a receiving channel including an intermediate frequencysection, means for automatically controlling the carrier frequency of asignal carrier transmitted through said intermediate frequency sectionduring signal reception, and means responsive to the conditioning ofsaid system for signal transmission for minimizing fluctuations in theoutput carrier frequency of said oscillator and for rendering saidlast-named means inactive.

16. In a combined radio transmitting and re-. ceiving system which isadapted to be selectively conditioned for signal transmission or signalreception, a transmission channel including a carrier producingoscillator; a receiving channel including an intermediate frequencysection, means for automatically controlling the carrier frequency of asignal carrier transmitted through said intermediate frequency sectionduring signal reception, means for controlling said oscillator tominimize fluctuations in the output carrier frequency thereof, and meansfor selectively rendering said two last named means operative to performtheir stated functions.

1'7. In a combined radio transmitting and receiving system which isadapted to be selectively conditioned for signal transmission or signalreception, a transmission channel including a carrier producingoscillator, a receiving channel including a mixer stage followed by anintermediate frequency section, a crystal controlled oscillator providedin said mixer stage, and means including said oscillator for minimizingfluctuations in the output frequency of said carrier producingoscillator when said system is conditioned for signal transmission.

18. In a combined radio transmitting and receiving system which isadapted to be selectively conditioned for signal transmission or signalreception, a transmission channel including a carrier producingoscillator followed by a mixer stage which includes a crystal controlledoscillator, a receiving channel including a mixer stage followed by anintermediate frequency section, a crystal controlled oscillator providedin the mixer stage of said receiving channel, and means including saidcrystal controlled oscillators for minimizing fluctuations in the outputfrequency of said carrier producing oscillator when said system isconditioned for signal transmission.

19. Transmitting apparatus comprising a first oscillator, meansincluding a second oscillator for converting the output voltage of saidfirst oscillator into a signal carrier voltage having a fre- 27 quencydifferent from the output frequencies of either of said oscillators, andmeans including a frequency discriminator controlled by said secondoscillator for maintaining the mean or center frequency of said firstoscillator substantially constant.

20. A wave signal transmission system comprising a first oscillator,means including a second oscillator for converting the output voltage ofsaid first oscillator into a signal carrier voltage having a frequencydifferent from the output frequency of either of said oscillators, andmeans including a frequency discriminator controlled by said secondoscillator for minimizing variations in the mean or center frequency ofsaid first oscillator.

21. In a combined radio transmitting and receiving system, a receivingchannel including a reception.

WILLIAM W. VOGEL.

